FIG. 1 is a schematic view of a magnetic resonant wireless electrical energy transmission device. A transmitting end is a resonant mechanism composed of a primary transmitting coil Ls and a resonant capacitor Cs. The primary transmitting coil Ls consists of a magnetizing inductance and a leakage inductance. A receiving end is a resonant mechanism composed of a secondary receiving coil Ld and a resonant capacitor Cd. To ensure effective transmission of wireless power, it is usually necessary to synchronize the resonant frequency of the primary and secondary resonant mechanisms and the system frequency. For example, Alliance for Wireless Power (A4WP) specifies the standards for 6.78 MHZ to enable the highest transmission efficiency. In this instance, T becomes a straight transformer T.
Meanwhile, to enhance the reception efficiency at the receiving end and suppress the interference of the magnetic field of the transmitting end to the charging equipment, the receiving coil is usually placed on a shielding layer composed of a magnetic sheet and a copper sheet. As shown in FIG. 2, as the magnetoresistance of the magnetic sheet is small, it attracts magnetic fields so as to undermine the magnetic field below the magnetic sheet. This mitigates the influence of the magnetic field on the charging equipment below the magnetic sheet. Meanwhile, the high frequency magnetic field causes a eddy current on the copper sheet and the counter magnetic field from the eddy current will further offset the inference of the magnetic field on the charging equipment. This approach maximizes the protection of charging equipment.
However, if the receiving coil and the transmitting coil fall in close proximity, the low magnetoresistance of the magnetic sheet makes it easy for the magnetic field of the transmitting coil to form a close loop, as shown in FIG. 3. This design shortens the magnetic lines of flux surrounding the magnetic field of the transmitting coil and hence magnifies the inductance value of the transmitting coil (Ls′). At this juncture, the compensating capacitor Cs and the primary coil Ls′ cannot resonate on the system's working frequency and the transmission efficiency will be affected.